YS-derived primitive macrophages are the major precursors of skin-infiltrating macrophages in E10.5 and E13.5 embryos. (A–C) Runx1Cre/wt : Rosa26R26R-eYFP embryos were treated with 4’OHT to induce Cre-mediated recombination at E7.25–E7.5 followed by analysis at E10.5 (A and B) or E13.5 (C). Controls are nontreated mice. (A) Flow cytometric analysis from one representative control or treated E10.5 embryo, showing the percentage recombination (eYFP+ cells) within CD45+CD11b+F4/80+ cells isolated from YS, brain rudiment, and limb buds at E10.5. (B and C) Percentage recombination within CD45+CD11b+F4/80+ cells isolated from the indicated tissues at E10.5 and E13.5. Error bars represent mean ± SEM of pooled data from 2 experiments (n = 8–16). (D and E) Developing skin (body and limb buds) was isolated from E10.0–E10.5 Ncx1−/− embryos or control littermates and processed for flow cytometric analysis. (E) Percentage ± SEM of hematopoietic cells (CD45+) in control littermates (white bars, n = 4) and Ncx1−/− embryos (black bars, n = 3). Pooled data from two independent experiments.